Wind Damage >> Wind Structural Design

Wind forces are also hazards to foundations. Wind pressure against the walls of an elevated house produce enormous strain on the superstructure-to-foundation Wind Structural Design connection. 

Further, poorly embedded pile-and-pier foundations are subject to racking (horizontal sheer force); Wind Structural Design lateral wind and water forces horizontally displace the superstructure from the foundation to a point at which the foundation "folds" beneath the superstructure. 

According to Murden (1991), wind-induced foundation failures were more common during Hugo than in other recent storms. Many homes on the South Carolina coast are elevated on masonry piers. These piers are often unreinforced and Wind Structural Design frequently exceed eight feet in height. Piers are usually on small individual spread footings. 

The net results of this design is piers that are unable to provide lateral support. As the supported structure moved from wind forces from Hugo, Wind Structural Design the piers underneath simply toppled. DAMAGE MITIGATION DURING CONSTRUCTION Pilkey et al. (1981) summarize safe coastal construction in one sentence: 

"Put the building or dwelling at a high enough elevation where the highest water will not reach it, and make it sturdy enough so the fastest winds will not destroy it." This recommendation should be applied to all residential Wind Structural Design construction in all hurricane hazards areas. 

Analysis of damage from Hurricane Andrew identified three factors that have the greatest effect on the hurricane resistance of a home: roof coverings, opening protection (windows and doors), and Wind Structural Design roof sheathing attachment. These three factors were consistently the weakest links in the homes examined (HUD, 1993). 

Hurricane-resistant building must focus on these weak links, but other potential failure areas such as walls and foundations should be addressed as well. Residential structures are best able to survive exposure to hurricane forces when the envelope of the structure consisting of exterior walls, roofs, and Wind Structural Design exterior opening closures remains intact as a unit. 

Pilkey et al. (1981) observed that buildings designed for inland areas are primarily designed to resist vertical (gravity) loads, and Wind Structural Design only insignificant wind forces. Homes in hurricane-prone areas, however, must be constructed to resist forces from many directions. Once any one building element fails, the other elements of the structure are at an increased risk. 

Further, not only must the individual structural elements remain intact, they must also provide unbreached load paths from one to the other. Roofs must not only be well constructed, Wind Structural Design but also adequately connected to walls. Walls in turn, must not only contain suitably strong windows and doors, but must also provide a load path from the roof to the foundation. 

Foundations must be capable not only of supporting the gravity load of the structure above them, Wind Structural Design but they must also be connected to the structure in a way that addresses flotation and horizontal loads, as well as transmission of those loads to the earth. Roofs 

When homes fail under exposure to hurricane-force winds, the order of failure is usually roofs, Wind Structural Design openings, and foundation (Perry, 1995). Roof systems are exposed to higher loading than any other building element (Smith and McDonald, 1991). 

Field observations of damage from hurricanes Hugo, Andrew, and Iniki confirmed that once the roof of a home was breached, failure of other building elements usually followed. Roof failure followed the following scenario. First, Wind Structural Design cladding was lost at a roof corner due to the greater uplift there, followed by loss of sheathing. 

Once sheathing was lost, the building envelope was effectively breached. Wind pressure was now exerted against the inside of the gable end-wall, leading to its possible failure, and Wind Structural Design against the underside of the remaining sections of the roof, increasing the likelihood that the reminder of the roof would be lost. 

If the roof became detached, gables collapsed, and the remainder of the structure, now much weakened, often failed. Roofs are subjected to wind forces from many directions. Direct wind pressure can loosen shingles and Wind Structural Design tiles. Suction forces on the surface of the roof and vortices on the roof corners can lift both roof cladding and sheathing. 

Internal pressure generated when windows, doors, or sections of the roof itself are breached can lift and separate the roof from the rest of the structure. A properly designed and Wind Structural Design constructed hurricane-resistant roof must be able to withstand all these forces. Hip vs. gable roofs 

Hip roofs, which slope in four directions, are less prone to breaching than gable roofs, Wind Structural Design which slope in two directions. HUD (1993) examined damage from Andrew and found that only 6% of houses with hip roofs were rated at the highest level of roof damage, while 33% of houses with gable roofs received this rating. 

This represents a significant amount of damage, since about 80% of the houses surveyed had gable roofs. Hip roofs do not present any flat surfaces to the wind despite wind direction, and Wind Structural Design the sloping faces of hip roofs enhance the performance of the roofing material (FEMA, 1986). 

Amirkhanian et al. (1993, Oliver and Hanson (1994), Pilkey et al. (1981), Riba et al. (1994), Wind Structural Design Sheffield (1993), and Keith (1994) all point to the superior performance of hip roofs. The hip roof generates much less uplift and is structurally better braced than a gable system (Sparks, 1994). 

Hip roof framing is effective structurally because it laterally braces the primary roof trusses, or rafters, and supports the top of the end walls of the home against lateral wind forces (Keith, 1994). Further, Wind Structural Design hip roofs eliminate the "hinge" formed between a gable end and gable-end wall. 

It was noted as early as Hurricane Camille in 1969 that hip roofs outperformed gable roofs (Pilkey et al., 1981). In 1986, Wind Structural Design FEMA noted that hip roofs appeared to perform best in high-wind areas. 

Amirkhanian et al. (1993) note that during Hugo, no hip roof building suffered more than 20% normalized direct wind damage, while among the gable-roofed homes, Wind Structural Design about 5% suffered direct wind damage of more than 25% of the insured structural value. Hip roofs were common in Hawaii and were found to have performed well during Iniki (Keith, 1994). 

Hip roofs performed well in South Florida as well. Unfortunately, in South Florida 80% of homes were constructed with gable roofs (Crandell et al., 1994). In hurricane-prone areas, both asphalt composition shingle and clay, and concrete tile, are common roofing materials, and Wind Structural Design both have proven problematic when exposed to hurricane-force winds. 

Asphalt shingle is warranted for wind gusts of 60 mph, although performance can be improved by sealing the edges at the roof eaves and rakes with roofing cement, and Wind Structural Design by using six nails per shingle rather than four nails or staples (Wolfe et al., 1994).

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